1. Field of the Invention
This invention relates to the application of Fischer-Tropsch chemistry to conversion of synthesis gas (hydrogen and carbon monoxide) to liquid hydrocarbons. In particular it relates to a Fischer-Tropsch reactor wherein the gases react in a slurry of catalyst powder suspended in molten wax. Such a slurry reactor has associated with it special problems in removing wax products from the reactor without removing fine catalyst particles as well.
2. Description of the Previously Published Art
In a slurry reactor in which a mixture of hydrogen and carbon monoxide are reacted on a powdered catalyst to form liquid hydrocarbons and waxes (Fischer-Tropsch reaction), the slurry is maintained at a constant level by continuously or intermittently removing wax from the reactor. The problem with wax removal is that catalyst in the wax must be separated from the slurry and returned to the reactor to maintain a constant inventory of catalyst in the reactor. Also, in order to keep the catalyst losses within the required replacement rate due to deactivation, the clarified wax removed from the system must not contain more than about 0.25% catalyst by weight. Several means have been proposed for separating the catalyst from the wax, e.g., centrifuges, cross-flow sintered metal filters, magnetic separators, etc.
The separation task is the most challenging when the catalyst produces free carbon and/or when particles break down during operation to produce "fines" which are sub-micron in size. In this case, it has been found that the small particles clog sintered metal filters to the point that back washing is ineffective. Also, centrifuges have been found unsuccessful in reducing the catalyst concentration below about 1% by weight in the clarified wax being removed.
Several methods have been described for separating catalyst particles from Fischer-Tropsch wax. A comprehensive report on the subject in entitled "Status Review of Fischer-Tropsch Slurry Reactor/Catalyst Wax Separation Techniques" prepared for the U.S. Department of Energy, Pittsburgh Energy Technology center by P. Z. Zhou, Burns and Roe Services Corporation, February, 1991. In this document are described filters, magnetic separators and settling devices, most of which were not successful or were not deemed commercially viable.
3. Objects of the Invention
It is an object of the invention is to provide an improved process for separating wax and catalyst whereby a relatively clean wax can be removed from the slurry reactor and the catalyst can be returned to the reactor without being subjected to attrition from a mechanical pump.
It is a further object of this invention to provide a catalyst particle separation device where the catalyst slurry obtains momentum as a jet as it issues from the feed conduit into the settler and where this momentum carries the catalyst particles in the settler in a direction opposite to that of the wax being removed from the settler.
It is a further object of this invention to provide a settler design where the combination of high upward velocities and a wire mesh filter within the settler enables the size and number of dynamic settlers to be reduced dramatically.
It is a further object of this invention to provide an expanded diameter section in a Fischer-Tropsch reactor which serves as a catalyst disengaging section so that the number of settlers required to remove wax of a specific clarity is reduced.
It is a further object of this invention to regenerate and increases the activity of a Fischer-Tropsch catalyst as well as to restore and maintain the selectivity of the catalyst by purging the catalyst with an inert gas for a period of time.
It is a further object of this invention to maintain the activity and selectivity of the catalyst more nearly constant over time in a slurry Fischer-Tropsch reactor by washing the catalyst with naphtha.
It is a further object of the invention to provide a means for using the settler return flow to impart an upward velocity to the slurry within the bubble column reactor thereby enabling larger catalyst particles to be used in the reactor. Larger catalyst particles enhance the performance of the dynamic settler.
It is a further object of the invention to provide a separate natural circulation conduit for recirculating a larger amount of slurry to the bottom of the reactor, whereby a larger upward velocity of the slurry in the reactor can be produced.
It is a further object of the invention to provide a separate liquid injection port at the bottom of the reactor whereby naphtha or other liquids from an outside source can be pumped into the reactor for imparting an upward velocity to the slurry in the reactor and for regenerating the catalyst. If olefinic Fischer-Tropsch naphtha is used then the naphtha can undergo additional chain growth to produce more diesel fuel.
These and further objects of the invention will become apparent as the description of the invention proceeds.